Extended Data Fig. 10. The OH to H ratio and the temperature trends for sulfur molecules produced by photochemistry.

a, $k_{{\mathrm{H}}_2\mathrm{O}}/k_{{\mathrm{H}}_2\mathrm{O}}^{\prime }\times \mathrm{O/H}$ as a proxy of OH to H ratio at 10× solar metallicity, in which $k_{{\mathrm{H}}_2\mathrm{O}}$ and $k_{{\mathrm{H}}_2\mathrm{O}}^{\prime }$ are the forward and backward rate constants of H₂O + H → OH + H₂, respectively. When OH becomes scarce relative to H as temperature decreases, the chain-forming path (does not require OH) is favoured over the oxidization path (requires OH). b, The average VMR between 10 and 0.01 mbar as a function of planetary equilibrium temperature with temperature profiles adopted from ref. ³⁹ (see text for the setup). The dotted grey line marks approximately the required SO₂ concentration to be detectable with WASP-39b parameters. S_x denotes the allotropes S₂ and S₈ and SO_x denotes the oxidized species SO and SO₂.